'\" et
.TH MQ_RECEIVE "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
mq_receive,
mq_timedreceive
\(em receive a message from a message queue (\fBREALTIME\fP)
.SH SYNOPSIS
.LP
.nf
#include <mqueue.h>
.P
ssize_t mq_receive(mqd_t \fImqdes\fP, char *\fImsg_ptr\fP, size_t \fImsg_len\fP,
    unsigned *\fImsg_prio\fP);
.P
#include <mqueue.h>
#include <time.h>
.P
ssize_t mq_timedreceive(mqd_t \fImqdes\fP, char *restrict \fImsg_ptr\fP,
    size_t \fImsg_len\fP, unsigned *restrict \fImsg_prio\fP,
    const struct timespec *restrict \fIabstime\fP);
.fi
.SH DESCRIPTION
The
\fImq_receive\fR()
function shall receive the oldest of the highest priority
message(s) from the message queue specified by
.IR mqdes .
If the size of the buffer in bytes, specified by the
.IR msg_len
argument, is less than the
.IR mq_msgsize
attribute of the message queue, the function shall fail and return an
error. Otherwise, the selected message shall be removed from the queue
and copied to the buffer pointed to by the
.IR msg_ptr
argument.
.P
If the value of
.IR msg_len
is greater than
{SSIZE_MAX},
the result is implementation-defined.
.P
If the argument
.IR msg_prio
is not NULL, the priority of the selected message shall be stored in the
location referenced by
.IR msg_prio .
.P
If the specified message queue is empty and O_NONBLOCK
is not set in the message queue description associated with
.IR mqdes ,
\fImq_receive\fR()
shall block until a message is enqueued on the message queue or until
\fImq_receive\fR()
is interrupted by a signal. If more than one thread is waiting to
receive a message when a message arrives at an empty queue and the
Priority Scheduling option is supported, then the thread of highest
priority that has been waiting the longest shall be selected to receive
the message. Otherwise, it is unspecified which waiting thread receives
the message. If the specified message queue is empty and O_NONBLOCK is
set in the message queue description associated with
.IR mqdes ,
no message shall be removed from the queue, and
\fImq_receive\fR()
shall return an error.
.P
The
\fImq_timedreceive\fR()
function shall receive the oldest of the highest priority messages
from the message queue specified by
.IR mqdes
as described for the
\fImq_receive\fR()
function. However, if O_NONBLOCK was not specified when the message
queue was opened via the
\fImq_open\fR()
function, and no message exists on the queue to satisfy the receive,
the wait for such a message shall be terminated when the specified
timeout expires. If O_NONBLOCK is set, this function is equivalent to
\fImq_receive\fR().
.P
The timeout expires when the absolute time specified by
.IR abstime
passes, as measured by the clock on which timeouts are based (that is,
when the value of that clock equals or exceeds
.IR abstime ),
or if the absolute time specified by
.IR abstime
has already been passed at the time of the call.
.P
The timeout shall be based on the CLOCK_REALTIME clock.
The resolution of the timeout shall be the resolution of the clock on
which it is based. The
.IR timespec
argument is defined in the
.IR <time.h> 
header.
.P
Under no circumstance shall the operation fail with a timeout if a
message can be removed from the message queue immediately. The validity
of the
.IR abstime
parameter need not be checked if a message can be removed from the
message queue immediately.
.SH "RETURN VALUE"
Upon successful completion, the
\fImq_receive\fR()
and
\fImq_timedreceive\fR()
functions shall return the length of the selected message in bytes and
the message shall be removed from the queue. Otherwise, no message
shall be removed from the queue, the functions shall return a value of
\-1, and set
.IR errno
to indicate the error.
.SH ERRORS
These functions shall fail if:
.TP
.BR EAGAIN
O_NONBLOCK was set in the message description associated with
.IR mqdes ,
and the specified message queue is empty.
.TP
.BR EBADF
The
.IR mqdes
argument is not a valid message queue descriptor open for reading.
.TP
.BR EMSGSIZE
The specified message buffer size,
.IR msg_len ,
is less than the message size attribute of the message queue.
.TP
.BR EINTR
The
\fImq_receive\fR()
or
\fImq_timedreceive\fR()
operation was interrupted by a signal.
.TP
.BR EINVAL
The process or thread would have blocked, and the
.IR abstime
parameter specified a nanoseconds field value less than zero or greater
than or equal to 1\|000 million.
.TP
.BR ETIMEDOUT
The O_NONBLOCK flag was not set when the message queue was opened, but
no message arrived on the queue before the specified timeout expired.
.P
These functions may fail if:
.TP
.BR EBADMSG
The implementation has detected a data corruption problem with the
message.
.LP
.IR "The following sections are informative."
.SH EXAMPLES
None.
.SH "APPLICATION USAGE"
None.
.SH RATIONALE
None.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "\fImq_open\fR\^(\|)",
.IR "\fImq_send\fR\^(\|)",
.IR "\fImsgctl\fR\^(\|)",
.IR "\fImsgget\fR\^(\|)",
.IR "\fImsgrcv\fR\^(\|)",
.IR "\fImsgsnd\fR\^(\|)",
.IR "\fItime\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "\fB<mqueue.h>\fP",
.IR "\fB<time.h>\fP"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
